Help!?

Hannah
I've been exclusively pumping since my baby was born and I've had a good supply and pumping about every 4 hours. Well the past few days I've noticed that my supply is getting less and less and I'm not doing anything different. Is this normal? Is there anything I can do to make it increase? I've been eating lactation cookies so I'm hoping that helps. 
Also, my baby will be 3 weeks this Friday!